Xenon flash tubes are mandated for use as anti-collision lights on aircraft. These tubes produce sudden, brilliant flashes of light that are much more conspicuous than other light sources. Current FAA (i.e., Federal Aviation Administration) mandated airworthiness standards require that such flash tubes have an effective intensity of 400 candela when viewed within 5 degrees of horizontal for aircraft certified after 1977. For aircraft certified before 1977 the requirement is 100 candela.
In a new strobe unit, the intensity of the xenon flash tube will meet or exceed FAA brightness standards. However, the intensity significantly degrades with use, long prior to actual tube failure. See: B. W. Henderson, "FAA: Aircraft Strobe Lights May Fall Short of Standards" Aviation Week & Space Technology, pp 42-43, Sep. 14, 1992; and V. M. Gardov, et al., "Theory of Powerful Nonsteady Xenon Discharge Taking Vaporization of Its Stabilizing Walls Into Account" translated from Teplofizika Vysokikh Temperatur, Vol. 19, No. 1., pp. 28-35, January-February, 1981. The result is that most flash tubes continue to work long after they have degraded below FAA minimum brightness requirements. An FAA survey showed that airlines generally rely on the technical manuals from the strobe light suppliers, which do not recommend checking brightness or regularly replacing xenon flash tubes. See Henderson, supra.
Strotek (Carson City, Nev.) claims to have developed a portable optical measuring system which can check flash tube intensity from outside the aircraft while they are on the ground. However, such a system is not an automatic self monitoring system, as disclosed and claimed herein.
U.S. Pat. No. 3,366,835 to H. L. Morris discloses a circuit for indicating a flash tube failure when the monitored tube is located where it is not readily visible to the operator. The failure indicator includes a light conducting plastic rod 48 which extends from the vicinity of the flash tube 14 to a photocell 49. Photocell 49 is part of a circuit including relay 53, capacitor 54, contacts 55 and warning light 56. If the flash tube fails to light for a predetermined period, capacitor 54 does not recharge and relay 53 is de-energized. Contact 55 then closes and the indicator light 56 comes on.
A similar problem is encountered with airport runway approach lighting systems. PAR-38 and PAR-56 tungsten-halogen incandescent lamps are currently used. The FAA requires that all lamps in a given system be of even brightness. However, the PAR lamps age with use, with the unacceptable result that some lamps appear dimmer than others.
A common solution to the foregoing problem with approach lighting systems is, per FAA regulations, to replace all the lamps in the system on a periodic basis (e.g., every 400 hours at major airports) even though many of the lamps still meet illumination requirements. Alternately, indirect monitoring systems are used which are subject to false indications caused by variances in loop current and lamp impedance, aging effects and by shorting devices intended to protect the system when a lamp fails. U.S. Pat. No. 5,105,124 to K. Futsuhara, et al., discloses a system in which a feedback signal consisting of a unique frequency for each lamp flows through the lamp circuit. Thus, any failed lamp can be detected by checking this feedback signal. Unfortunately, this system does not detect lamps which have not failed, but are below FAA brightness requirements.
Numerous other lamp failure indicators are disclosed in the prior art. See, for instance: U.S. Pat. No. 3,541,504 to R. H. Bush, which is described as a vehicle burn-out indicator; U.S. Pat. No. 3,588,816 to R. H. Himes; U.S. Pat. No. 3,624,629 to C. A. Donaldson; U.S. Pat. No. 4,572,987 to D. M. Embrey, et al.; and U.S. Pat. No. 4,376,910 to J.P. Pieslier.
In view of the foregoing, it is an object of the present invention to provide systems for the continuous monitoring of flash tubes and continuous illumination sources, which systems provide a fault signal or other indication when the intensity of the source being monitored falls below a predetermined (sometimes mandated) value (e.g., a minimum set by a regulatory agency such as the FAA).
It is also an objection of the invention to improve airport runway approach lighting by providing a system capable of monitoring, either continuously or on command, each of the lamps used therein and providing a fault signal or other indication when the intensity falls below the required minimum (even though the lamp has not failed).
It is a further object of the invention to use the existing runway approach lamp power grid to avoid the requirement for any additional wiring.
It is another object of the invention to provide a system which can be used to monitor both high intensity constant current and medium intensity constant voltage runway approach lighting systems.
The foregoing improves both aircraft and runway safety and, at least in the case of runway approach lights, reduces the unnecessary replacement of lights in those systems where, for lack of applicant's monitoring system, all lamps are replaced on a periodic basis regardless of their individual intensities. In addition, the monitoring capability can statistically record average bulb lifetimes, thus allowing an accurate forecasting of bulb purchases.